The present invention relates to a new and improved construction of an electronic protective circuit.
Generally speaking, the electronic protective circuit of the invention serves for interrupting the current flowing from a bridge rectifier or a centre point circuit to an inductive load and for deenergizing the inductive load. This protective circuit contains an extinguishing circuit which is connected in parallel with at least one switching thyristor and an extinguishing thyristor and a capacitor connected in series therewith. Further, there is provided a deenergization circuit connected in parallel with the load and containing at least one deenergization resistance.
An electronic protective circuit, contemplated for use with direct-currents, of the aforementioned type is already known and has been illustrated and described, for instance, by K. Heumann in the publication Wiss. Ber. AEGTELEFUNKEN, Volume 48 (1975), page 115. This protective circuit contains a diode connected parallel to the load and polarized towards the load current. The diode forms a deenergization circuit which after cutoff of the load current shunts the inductive load and carries the deenergization current. If this protective circuit is used for mixed or complex currents of a controlled rectifier, which with large control angle has its voltage experiencing a brief reversal of the polarization, then during this time span the diode is switched into its conductive state and forms a shunt with respect to the load through which there can flow part of the load current.
In order to avoid such current shunt there have been used for large electrical machines, whose field winding is connected with a multi-phase rectifier, preferably protective switches with explosive triggering. The construction and mode of operation of such protective switches are well known to those skilled in the art and, for instance, have been illustrated and disclosed by Bruckner and Schmitz in Kalor-Emag-Mitteilungen 1956, Volume 4, pages 2 et seq. Although this type of switch functions satisfactorily it still is afflicted with a number of appreciable drawbacks. The switch is destroyed each time that there is undertaken the explosive triggering and thus must be replaced, it requires a massive, relative large housing which entraps parts which are propelled away during the explosion operation, and it cannot be installed in moved machine parts.